Elastic tissues in series and in parallel with muscle have a profound influence on muscle mechanics and energetics. Planar elastic tissues including aponeurosis and fascia are intimately connected with muscles and only recently have we begun to understand the extent of their influence on muscle force, speed, and power. By forming a sheath around a large portion of the muscle belly, aponeurosis is hypothesized to govern the shape changes that muscle undergoes during contractions but this has never been directly tested. Through this influence on muscle shape, the aponeurosis may modulate the speed and force of contraction. Changes in connective tissue stiffness occur with age and disease and may interfere with the aponeurosis' ability to match muscle shape changes to muscle demand, which could have detrimental effects during damaging lengthening contractions. By modifying the structure and stiffness of the aponeurosis, this project will directly examine the role of aponeurosis in modulating muscle shape changes and test how similar alterations occurring after surgical intervention or with injury, age, or disease influence the mechanics of muscle contraction. This study will also explore the link between altered aponeurosis stiffness and muscle damage to address whether changes in aponeurosis properties related to age or disease increase the muscle's susceptibility to injury. The proposed experiment will use a novel 3D imaging technique to visualize the dynamic changes in muscle shape, muscle length, and fiber length in an in situ muscle preparation in a turkey. The specific aims of this project are: 1 to determine the role of the planar structure of aponeurosis in maintaining intramuscular pressure and modulating muscle force and speed during contractions; 2) to determine the role of aponeurosis stiffness in modulating muscle force and speed and preventing muscle damage. The results of the proposed experiments have the potential to improve surgical outcomes and inform rehabilitative strategies targeting connective tissue properties after injury or disease. Th proposed work will form the basis for future research investigating how planar connective tissues integrate function within and among muscles and between limb segments and incorporating planar connective tissues into musculoskeletal models of human movement to further explore how modifications of connective tissue properties related to injury, age, disease, or surgical intervention alter muscle and locomotor function.